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Simple Cub - scratch build but way tail heavy?

#1
Hi - pretty happy with the Simple Cub as a build but putting the motor and battery in and discovered that in standard position for the motor with the battery full forward the tail won't even leave the bench with a CG test. What's going on here? Is it because the foam board I use is heavier than the FT stuff? My marked CG is 45mm from the leading edge per the data sheet. At this rate I would have to have the motor sitting 10cm forward to get any kind of balance. Didn't expect it to be that far out. Using a 1500mah.
 

Bricks

Well-known member
#4
I have built a few plane from Bordens foam which is a lot heaver then Dollar Tree foam. I knew it was going to be tail heavy so I did every thing I could to keep weight any weight closer to the CG. Moved the servos into the fuselage closer to the CG, pulled the inside paper, used less of a finish on the tail, kept the hot glue to a minumum, moved esc and battery as far forward as possible, even wing servos I moved farther forward in the wing.. Still had to add some lead to the nose but not near as much if I had not done all of the above..

Building light flies way better then heavy but some times it gets away from us.
 

Merv

Well-known member
#5
I have had to add 2 inches to the nose on of some FT planes to get the plane to balance. Rather than recreate the nose, I cut it off, just ahead of the front window, add in the foam board needed and glue the nose back on.
 
#6
Well, thanks everyone but am disappointed to announce that this plane ended up being too heavy to fly at all! It just crawled along the grass, sputtered to about 6 ft off the ground and wobbled around before hitting the deck. I did think this might happen though. Fully laden, it was 1.1 kg which for a plane the size of the Simple Cub seemed very excessive, really felt like a brick with wings. So not sure what people are building these with, but obviously different materials to me. I am not going to mend this one. Sad, because it did look cool!
 
#10
Actually, jokes aside, I started to think about the power issue and whether to upgrade the motor. I started to look into it but there seems to be so many factors in this area: prop size, ESC, battery, not to mention trying to work out what you need in a motor and the terminolgy (eg KV etc) and what this means to the plane you've built. So if anyone has a simple equation to tell me what kind of motor and prop will suit a seemingly heavy 1.2 kg cub (flying weight) with a 900mm wingspan AND kindly tell my WHY you know it would work, I'd be interested to hear it. Or maybe it's not worth bothering with and build something else? Can a motor be too big, for example when it's additonal weight is not going to give the plane any additional performance. Sorry for the semi noob questions. Cheers
 

Tench745

Well-known member
#12
Actually, jokes aside, I started to think about the power issue and whether to upgrade the motor. I started to look into it but there seems to be so many factors in this area: prop size, ESC, battery, not to mention trying to work out what you need in a motor and the terminolgy (eg KV etc) and what this means to the plane you've built. So if anyone has a simple equation to tell me what kind of motor and prop will suit a seemingly heavy 1.2 kg cub (flying weight) with a 900mm wingspan AND kindly tell my WHY you know it would work, I'd be interested to hear it. Or maybe it's not worth bothering with and build something else? Can a motor be too big, for example when it's additonal weight is not going to give the plane any additional performance. Sorry for the semi noob questions. Cheers
Adding power to fight a heavy airplane can be like chasing your own tail.
Your plane is too heavy for the power available. =>
You add a bigger motor. =>
The big motor uses more power. =>
To keep the same flight time you add a bigger battery. =>
Bigger battery and motor means more weight. =>
Your plane is too heavy for the power available. =>

These are the kinds of things almost everyone has to work through as they learn what does and doesn't work and why.
Knowing what motor, prop, ESC, and battery you have now as well as how heavy the empty model is, etc. would be useful in solving some of your conundrum.
 
#13
You could try and run it on 4S if your motor and ESC are rated for it. You did not mention what motor you have in it now?
Thanks - I had a Dualsky 2312C V2 in it, Peak Power 258W? Powered by 2200 mah 3S running a 10x5 E. Since that could barely get it to take off (including hand launch) I have put an E Flight 15BL EFL5367 1050KV now running an 11 x 7E. Same battery. Just took it up, flies great actually. I guess I was curious if there was a simple ball-park formula to indictate the required motor/prop for a given weight?
 
#14
Adding power to fight a heavy airplane can be like chasing your own tail.
Your plane is too heavy for the power available. =>
You add a bigger motor. =>
The big motor uses more power. =>
To keep the same flight time you add a bigger battery. =>
Bigger battery and motor means more weight. =>
Your plane is too heavy for the power available. =>

These are the kinds of things almost everyone has to work through as they learn what does and doesn't work and why.
Knowing what motor, prop, ESC, and battery you have now as well as how heavy the empty model is, etc. would be useful in solving some of your conundrum.
Ah ok, I guessed that would occur. I suppose the moral of the story is to build them light in the first place. I suspect actually I did not give enough credit for the weight of the paint! This was black foam board and it used over a full can of yellow. That might have done it!
 

Merv

Well-known member
#16
So if anyone has a simple equation to tell me what kind of motor and prop will suit a seemingly heavy 1.2 kg cub (flying weight) with a 900mm wingspan
Great looking plane.

Here are some guidelines to get you started in the right direction. Your flying stile will dictate the power requirements.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

Another way to approach the issue is from prop thrust/ total weight of the plane. A prop thrust that is 60-70% of the weight of the plane is minimal. 1 to 1 is very good. 1.5-2 to 1 is 3D.
 
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#18
Great looking plane.

Here are some guidelines to get you start in the right direction. Your flying stile will dictate the power requirements.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

Another way to approach the issue is from prop thrust/ total weight of the plane. A prop thrust that is 60-70% of the weight of the plane is minimal. 1 to 1 is very good. 1.5-2 to 1 is 3D.
Thanks Merv, that's a great summary and very helpful.
 
#20
image.jpg

This is my simple cub. same exact issues guess I will power up. The good thing is I am building one with my Daughter while we are in isolation. Will do everything I can to make it nose heavy. Hopefully that’ll help